Frequency conversion system



1941 F. M. DEERHAKE 2,228,815

FREQUENCY couvznsxon SYSTEM Filed Jam 27, 1940 I 6 7 /2 Is 2 f I I I I 1f2 CONVERTER AMPLIFIER coIvvERrER AMPLIFIER g f5 /o BUFFER AMPLIFIER fAMPLIFIER II s MASTER BUFFER f OSCILLATOR AMPLIFIER 8 I HARMONIC -4GENERATOR CONSTANT FREQUENCY 3 Osc/LLATOR lnveht 01*: Franklin M. Deer"h a ke Patented Jan. 14, 1941 UNITED STATES FREQUENCY CONVERSION SYSTEMFranklin M. Deerhake', Schenectady, N. Y., assignor to. General ElectricCompany, a corporation of New York Application January 27, 1940, SerialNo. 315,998

10 Claims. (01. 2 50-20) My invention relates to a frequency conversionsystem of the double heterodyne conversion type, particularly adaptedfor converting signal-modulated waves from one high frequency to anotherhigh frequency.

The advantages of the double heterodyne conversion system, whichcomprises a first heterodyne conversion to an intermediate frequency atwhich the signal-modulated wave is readily am- 10 plifled, followed by asecond heterodyne conversion to the desired resultant frequency, arewell known. Such a system requires two locally generated waves 'ofdifferent frequencies for effecting the consecutive conversions. Twoindependent oscillators may be employed or the two Waves may comprisedifferent harmonics of a particular frequency derived from a singleoscillator. The

former case permits greater flexibility in the choice of frequenciesthan the latter, but ordinarily the frequency of the resultant outputwave is affected by any fluctuations in thefrequencies of the twolocally generated waves in a random manner. Consequently, the advantagesof greater flexibility may be ofiset either by decreased 25 frequencystability of the output wave or by the expense of providing oscillatorsof closely controlled frequency.

It is an object of my invention to provide an improved double heterodyneconversion system which employs two separate oscillators and which hasboth flexibility and desirable characteristics.

Another object of my invention is the provision of such a frequencyconversion system adapted for stable operation at very high frequencies.

Another object of my invention is to provide an economical and highlystable double heterodyne conversion system in which the waves generatedby one oscillator can be utilized directly in one of the conversions,thereby eliminating the need for intervening frequency multipliers orlike devices which may introduce undesirable interfering frequencies.

It is a further object of my invention to provide a double frequencyconversion system in which the waves employed for effecting theconsecutive conversions are interdependent though derived fromindependent sources.

Another important object of my invention is system in which fluctuationsin the frequency of one of the locally generated waves may becompensated so as to have'no effect on the frequency of the resultantwaves.

55 The features of my invention which I believe the provision of adouble heterodyne conversion to be novel are set forth withparticularity in the appended claims. My invention itself, however, bothas to its organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in connection with the accompanyingdrawing, in which the single figure diagrammatically illustrates oneembodiment of my invention.

Referring now to the drawing, the invention is shown embodied in a radiorelay station for receiving signal-modulated waves of one frequency andretransmitting or rebroadcasting similarly modulated waves of adifferent frequency. Since the component elements of the system are allconventional and well known to those skilled in the art, the circuitconnections and elements have not been shown in detail but arerepresented in conventionalized form by a one line diagram.

High frequency waves, which may be modulated in any desired manner, asby audio signals or television video signals, are received by a suitableenergy reception device such as the antenna I at the left-hand side ofthe drawing. Waves of a different high frequency, similarly modu1ated,-are to be retransmitted or rebroadcasted by a suitable energypropagation device, such as the antenna 2 at the right-hand side of thedrawing. 'It is of course obvious that the waves may be received andretransmitted by wire lines rather than by the wireless system shown. Alocal oscillator 3 generates a substantially constant frequency waveutilized in one of the frequency conversions, to be described presently.This oscillator is of conventional design, and frequency stability maybe insured in any wellknown manner, as by crystal or resonant linecontrol. For the particular embodiment shown, representing a highfrequency relay station, the frequency of the oscillator 3 willordinarily be too low to be used directly, and hence a synchronizedharmonic generator 6 is employed, operating at some fixed multiple ofthe frequency of oscillator 3.

The oscillator employed in this system for producing the second waveutilized in the frequency conversions is the master oscillator 5. Aswill be explained in detail later, .the frequency'of this oscillatorordinarily will not be highly constant but may fluctuate, eitherpurposely or through inherent instability. Waves generated by the masteroscillator 5 are heterodyned with the received signal-modulated wave inthe converter 8 and a selected modulation product resulting from,

thisconversionissuppliedtctheamplifier'l. The converter 8. often calleda detector or mixer, may comprise any one of a number of conventionalcircuits well known in the art, and it is thought unnecessary todescribe it in detail. Briefly, the converter performs the two functionsof heterodyning the two waves to producers group of modulation productsand of selecting one desired product as the converter output wave. Wavesgenerated by the oscillator 5 are also heterodyned with waves suppliedby the harmonic generator 4 by means of a converter I, which functionsin the same manner as the converter i, and a selected modulation productis supplied to the amplifier I. Buffer amplifiers l0 and II arepreferably interposed between the master oscillator I and the converters8 and t to amplify the oscillations and prevent undesirable reactionsbetween the various circuit elements, according to known practice.

The signal-modulated waves supplied to the amplifier I, as a result ofthe first conversion, are next subjected to a second conversion in theconverter I! in which they are heterodyned with the waves from theamplifier 9. The selected modulation product of this conversioncomprises the signal-modulated output wave which is supplied to theantenna 2. Optionally, it is first further amplified by the amplifier l3.prior to retransmission.

It will be understood, of course, that one or more of the variousamplifiers shown may be omitted, or that additional amplifiers may beemployed where desirable as, for example, between the antenna l and theconverter 6. Since frequency stability of the waves generated by masteroscillator is not a primary consideration, the waves may be generateddirectly at the desired frequency. No frequency multiplying device, suchas a harmonic generator similar to the harmonic generator [employed withconstant frequency oscillator 3, will generally be required even athigher frequencies. This is a distinct advantage in that it reduces thecost and complexity of the apparatus and also minimizes the possibilitythat undesired parasitic frequencies will be introduced into thecircuit, as will be apparent to those skilled in the art.

An important feature of my invention is the mannerin which thefrequencies of the waves are related. Referring to the drawing, the wavereceived at the antenna l is designated by f1, indicative of itsfrequency. Similarly the output wave at antenna 2 is denoted by h, theconstant frequency wave supplied by theharmonic generator l by Is, thewave supplied by the master oscillator 5 by ii, the selected modulationproduct supplied to amplifier 1 by Is and the selected modulationproduct supplied to amplifier O by fa.

The output frequency f: is determined by the values of f1 and either faor it, depending on the particular combination of modulation productsselected in the three conversions. First assume, as is usually the case,that ii and I: are arbitrarily selected, and that is is to remainsubstantially cons n Fr quency is is then selected so that I: is equaleither to the sum or to the difference of 11 and fa. If it is selectedequal to the sum, obviously i: must be greater than f1, whereas if it isselected equal to the difference, I: may be either greater or less thanf1.

'Now under the above-assumed conditions, for any chosen value of 14 aparticular combination of the sum or difference frequency modulationsmears products will produce the desired value of is. There are a numberof combinations which will give 1:, determined by the relativemagnitudes of [1, I: and f4 and on the choice of modulation products.

Under these conditions it is desired to investigate the effect ofvariations in master oscillator frequency 14 upon the output frequencyis. This effect will depend upon the relative magnitudes of 11, f: andf4 and upon the particular modulation'products is and fa selected.However, for any possible combination a variation in is will produce oneof two results: either the frequency variation in {4 will not vary j: atall, or it will produce twice this frequency variation in {2. Thus thefrequency variation in h will either be compensated completely ormultiplied as far as its effect upon I: is concerned.

In some situations it may be desirable to utilize the frequencymultiplication effect just described, but in the particular embodimentillustrated it is desired to utilize the frequency compensation effect.As stated, there are a number of different combinations of the sum or'difference frequency modulation products is and fa which will producethis compensation effect. In general, there are two possiblecombinations of these principal modulation products for any selectedvalues of f1, f3 and f4. It can also be stated as a general rule that ifthe selected modulation products is and fa" vary in the same sense asvariations in it, then I: must be their diflerence frequency product;whereas, if the selected modulation products It and fa vary in theopposite sense with variation in {4, then I: must be their sum frequencyproduct. If it is desired to utilize the frequency multiplicationeffect, obviously the converse .is true.

To illustrate these rules more clearly tables are presented below ofsome possible combinations of frequencies which will eliminate theeffect of variations in master oscillator frequency 14 upon the outputfrequency In. Table I im mediately below illustrates various frequencyrelationships which will maintain I: independent of variations in ft forthe case where f: has been selected equal to the difference between 11and f3.

Thus in the first case above, when it is either greater than or lessthan both If and is. is may be selected as the sum frequency product ofi1 and f4, fs as the sum frequency product of I: and f4, and f: as thedifference frequency product of f5 and is; or It may be selected as thedifference frequency product of f1 and f4, Is as the differencefrequency product of f: and f4, and f: as the difference frequencyproduct of f5 and is.

Table 11 below illustrates various frequency relationships which willmaintain is independent of variations in ft for the case where I: hasbeen selected equal to the sum of f1 and f3.

aaaasis Summarizing the above if f1, f5 and' one' oscillatorfrequencyare selected, then for any bination of modulation products canbeselected to give the desired result. For example, in an actualinstallation Of the system shown in the drawing it was desired toconvert a band of signal-modulated waves having a carrier frequency of157.25 me. to an identicalband having a carrier frequency of 67.25 mc.The combination of frequencies indicated by asterisks in Table I abovewas selected. The actual values of the various frequencies were asfollows:

f1=157.25 mc. f2=67.25 mo. f =90 mc. f4=123375 mc. j =28.5 mo.

fer-38.75 me.

It will be seen that I have provided a simple frequency conversionsystem having the flexibility of the double heterodyne system employingindependent oscillators while retaining the fre= quency stability of thesingle oscillator system.

While I have shown one form of my invention embodied in a radio relaystation, it will of course be understood that I do not wish to belimited thereto. As previously mentioned, there may be occasions when itis desirable to utilizethe frequency multiplication effect which can beobtained by suitable combinations of selected modulation products. Thus,the frequency of the master oscillator 5 may be varied purposely inorder to frequency modulate the output wave by twice that amount.Furthermore, by selecting modulation products in the three; conversionswhich are higher harmonics of'the combining frequencies, rather than theprincipal sum or difference frequency products, the frequencymultiplication efiect can be "amplified to give a frequency variation ofthe output wave greater than twice the frequency variation inLettersPatent of theUnited States is:

1 The method of converting-electrical waves frequencies to oscillator I.The tables above illustrate only combinations of principal modulationproducts giving the frequency compensation effect. It is not thoughtnecessaryto enumerate combinatime producing the converse effect sincethese are thought to be obvious from the above de- -scription.- Sincethis and many othermodiflcati'onsfmay be-m'ade, bdh in the circuitarrange- I .ment and instrumentalities employed, I contemplate bythe-appended claims to coveranysuch modifications as fall, within thetrue spirit. and

- scope'ofniy inventionl" I What I'claim asnew and desire to secure byof a first frequency to waves of a second frequency which comprisesthesteps offp'roducing waves. of a, thirdfrequency, producing waves of afourth frequency, combining the waves of said first'and-thirdfrequencies to produce a resultant f. frequency, combiningflthe waves ofsaid 1 third value of the other oscillator frequency a com-Q and fourthfrequenciesito produce a second resultant frequency, and combining saidresultant produce said second frequencywaves. r

2. The method of frequency conversion which comprises the steps ofreceiving signal-modulated waves of a first frequency, generating wavesof a second, substantially constant frequency, --generatingindependently. waves of a third frequency subject to frequencyvariations, hetero- ,dyning the waves of said first and thirdfrequencies to produce a first modulation product the frequency of whichis affected by said variations, heterodyning the waves of said secondand third frequencies to produce a second modulation product thefrequency of which is'also affected bysaid frequency variations andheterodyning said modulation products to produce a resultant wave thefrequency of which is unaffected by said variations.

3. In a frequency conversion system for converting electrical waves of afirst frequency to waves of a second frequency, the combination of anoscillator for generating waves of a third frequency, a secondoscillator for generating independently waves oi a fourth frequency, aconverter for combining the waves of said first and fourth frequenciesand for selecting adesired modulation product, a converter for combiningthe waves of said third and fourth frequencies and for selecting asecond desired modulation product, and a converter for combining saidselected modulation products and for selecting as a resultant modulationproduct waves of said second frequency.

4. A system for theconversion of signal-modulated waves of a first highfrequency to similarly modulated waves of a second high frequencycomprising means for receiving the waves of said first frequency, asource of waves of a third high frequency subject to frequencyvariations, means for heterodyning the waves of said first and thirdfrequencies to produce waves of a resultant frequency, means forutilizing said resultant frequency wavesto produce the waves of saidsecond frequency, and means for preventing frequency subject tofrequency fluctuations, a converter for heterodyning the waves of saidfirst and third frequencies to produce a modulation product thefrequency of which is affected by said fluctuations, ,a second converterfor heterodyning the waves of said second and third frequencies toproduce a second modulation product the frequency of which is alsoaffected by said 'ing one ference frequency products and for selectingone product affected by said variations in one sense, a second converterfor combining the waves of said third and fourth frequencies to producesum and difference frequency products and for selectproduct affected bysaid variations in the oppositesense. and a third converter forcombining said selected products to produce sum and fluctuations, and athird converter for heterodyn-/ ;diiferene frequency products and forselecting ing said modulation products to produceare- 'sultant wave thefrequency of which is unaffected by said fluctuations.

6. In a system for converting electrical waves their sum frequencyproduct as said second high frequencywaves.

9. In a frequency conversion system for converting received,signal-modulated waves of a of a first frequency to waves of a second -fre-" first high frequency to similarly modulated waves quency, thecombination of a source of waves of a of a second high frequency, thecombination of third frequency, a source of waves of a fourth anoscillator for generating waves of a third, subfrequency, means forcombining the waves of said first and fourth frequencies to produce aresultant frequency, means for combining the waves of said third andfourth frequencies to produce a second resultant frequency, and meansfor combining said resultant frequencies to produce said secondfrequency waves.

'1. The combination in a frequency conversion system, of means forreceiving waves of a first frequency, means for generating waves of asecond, substantially constant frequency, means for generatingindependently waves of a third frequency subject to frequencyvariations, means for combiniig the waves of said first and thirdfrequencies to produce a pair of sum and difference frequency productsthe frequencies of which are affected by said variations and forselecting one product from said pair, means for combining the waves ofsaid second and third frequencies to produce a-second pair of sum anddifference frequency products the frequencies of which are also affectedby said variations and for selecting one product from said second pair,means for combining said selected products to produce a third pair ofsum and difference frequency products and for selecting as a resultantwave one product of said third pair the frequency of which is unaflectedby said variations.

8. In a frequency conversion system for converting received,signal-modulated waves of a first high frequency to similarly modulatedwaves of a second high frequency, the combination of an oscillator forgenerating waves of a third, substantially constant high frequency, asecond oscillator for generating waves of a fourth high frequency.subject to frequency variations, a first converter for combining thewaves of said first and fourth frequencies to produce sum anddifstantially constant high frequency, a second oscillator forgenerating waves of a fourth high frequency subject to frequencyvariations, a first converter for combining the waves of said first andfourth frequencies to poduce sum and difference frequency products andfor selecting one product affected by said variations in one sense, asecond converter for combining the waves of said third and fourthfrequencies to produce sum and difference frequency products and forselecting one product affected by said variations in the same sense, anda third converter for combining said selected products to produce sumand difference frequency products and for selecting theirdifferencefrequency product as said second high frequency waves.

10. A radio relay station for receiving signalmodulated waves of a firsthigh frequency and for retransmitting similarly modulated waves of asecond high frequency, comprising, in combination, a first oscillatorfor generating waves of a third substantially constant high frequencyequal to the difference between said first and second frequencies, asecond oscillator for generating waves of a fourth high frequencyintermediate said first and third frequencies and subject to frequencyfluctuations, a first converter for heterodyning together the waves ofsaid first and fourth frequencies and for selecting their differencefrequency product, a second converter for heterodyning together thewaves of said third and fourth frequencies and for selecting theirdifierence frequency product, a third converter for heterodyningtogether said selected products and for selecting their sum frequencyproduct as said second high frequency waves and means for retransmittingsaid second high frequency waves.

FRANKLIN M. DEERHAKE.

